1. Field of the Invention
This invention generally relates to an air-fuel ratio controller for an internal combustion engine. More specifically, the present invention relates to an air-fuel ratio controller having a catalytic converter.
2. Background Information
A catalytic converter is an emission control device in an exhaust stream that chemically treats or purifies exhaust gas discharged from an internal combustion engine of a vehicle. Once a mixture of air and gas is fed into cylinders of the internal combustion engine, the mixture is subjected to combustion and explosion within the cylinders. Then, the gas is discharged from the cylinders as an exhaust gas, which contains environmental pollutants such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxide (NOx). The catalytic converter is disposed in an exhaust passage to purify such pollutants.
It is well known to utilize a catalytic converter with a three-way catalyst in the exhaust passage to remove HC, CO, and NO.sub.x, from the exhaust gas. The catalyst of the three-way catalytic converter oxidizes HC and CO, and deoxidizes NO.sub.x. For the three-way catalytic converter to be able to perform both oxidization and deoxidization efficiently, the air-fuel ratio of the exhaust gas needs to be in the vicinity of the stoichiometric air-fuel ratio. Since three-way catalytic converters are well known in the art, the structure and function of a three-way catalytic converters would be obvious to one of ordinary skill in the art without further explanation herein. An air-flow controller or carburetor is a device that controls the air-fuel ratio. In the air-flow controller, an air-fuel ratio sensor has been arranged in the exhaust passage on the upstream side of the three-way catalytic converter to detect the air-fuel ratio of the exhaust gas.
Generally, the air-fuel ratio of the exhaust gas becomes greater than the stoichiometric air-fuel ratio (lean air-fuel ratio) during acceleration of the vehicle due to an increase in air intake. Conversely, the air-fuel ratio becomes smaller than the stoichiometric air-fuel ratio (rich air-fuel ratio) during deceleration of the vehicle. To allow the three-way catalytic converter to operate efficiently even when the air-fuel ratio of the exhaust gas fluctuates, it has been proposed to utilize a three-way catalytic converter having the ability to store oxygen (O.sub.2). In this arrangement, the three-way catalytic converter has an oxygen absorbent material for storing oxygen in the catalytic converter. The quantity of oxygen to be stored in the three-way catalytic converter is calculated based on the deviation of the air-fuel ratio of the exhaust gas from the stoichiometric air-fuel ratio. Further, as disclosed in Japanese Laid-Open Japanese Patent Publication No. H6-249028, the air-fuel ratio of the intake air-fuel mixture is controlled so that the estimated amount of oxygen to be stored in the three-way catalytic converter matches a target value, such as roughly half the oxygen storage limit value of the oxygen absorbent material of the three-way catalytic converter.
In this catalytic converter, when the air-fuel ratio of the exhaust gas is greater than the stoichiometric air-fuel ratio, i.e., a lean mixture, oxygen adsorbs onto the oxygen absorbent material of the three-way catalytic converter. Conversely, when the air-fuel ratio of the exhaust gas is smaller than the stoichiometric air-fuel ratio, i.e., a rich mixture, oxygen desorbs from the oxygen absorbent material of the three-way catalytic converter. Since the rate at which oxygen desorbs from the three-way catalytic converter is slower than the rate at which oxygen adsorbs onto the three-way catalytic converter, it has been proposed in Japanese Laid-Open Japanese Patent Publication No. 9-310635 to adjust the quantity of oxygen to be stored based on whether the oxygen is adsorbed or desorbed.
However, with this kind of device, when a cylinder of the engine misfires, unburned fuel and air enter the three-way catalytic converter. Consequently, the air-fuel ratio sensor cannot measure the air-fuel ratio accurately, and also the quantity of oxygen stored in the three-way catalytic converter cannot be accurately computed.
Basically, when misfiring occurs, if the quantity of oxygen to be stored continues to be calculated based on the output of the air-fuel sensor, a gap will develop between the computed value and the actual value of the quantity of oxygen stored in the three-way catalytic converter. Accordingly, the air-fuel ratio will be controlled based on an incorrect value.
In view of the above, there exists a need for an air-fuel ratio controller which overcomes the above mentioned problems in the prior art. This invention addresses his need in the prior art as well as other needs, which will become apparent to those killed in the art from this disclosure.